Solutions have already been proposed to make the technical interface consisting of the electrodes as discrete as possible and not to detract from the aesthetics of the electronic device, in particular in the case of a timepiece. For example, there are prior art wristwatches in which the inside face of the glass carries touch-sensitive electrodes for controlling time functions or other functions by means of a capacitive or resistive effect, as described in a non-limiting manner in the patents U.S. Pat. No. 4,228,534, EP 0 674 247 and EP 1 207 439. The glass can also be replaced by or have added to it a cell formed of two substrates with transparent electrodes between which there is an active material, for example to form a photovoltaic cell constituting a power supply, as described in the document WO 93/19479, or to form a liquid crystal cell that can have either a transparent state or a state for displaying on demand information complementary to or different from that displayed on an underlying dial, as described in the document WO 99/32945.
Transparent conductive oxides (TCO), such as indium and tin oxide (ITO), In2O3 and SnO2 doped with antimony, used in the prior art to produce the electrodes are conductive and also transparent in the visible spectrum. These materials are deposited to a thickness of 50 to 100 nm directly onto the transparent substrate or onto an intermediate layer, using any of a large number of techniques known in the art, such as spraying, evaporation, the sol-gel technique, and chemical vapour deposition (CVD) techniques, in particular the light-induced (laser-assisted) chemical vapour deposition (LICVD) technique. With regard to the structure of the electrodes, various methods used in the prior art employ at least one mask corresponding to the contour of the electrodes, either during deposition of TCO by localised crystallisation of a sol-gel film by irradiation with a UV laser or by applying to a continuous TCO film either chemical etching or localised ablation by irradiating it with UV radiation of sufficient fluence. The nature of the transparent substrate (glass or plastics material) is obviously vital from the technical and economic points of view to the choice of the process to be used. For example, localised crystallisation of a sol-gel film by a UV laser cannot be applied to a plastics material (for example PMMA) substrate, because this involves a photothermal process.
At normal incidence, a light ray passing through the TCO of refractive index n1 and the substrate of refractive index n0, or only the substrate, is not deflected and the electrodes are therefore invisible. On the other hand, at oblique incidence, the light path is modified, because of the different values of n0 and n1, with the result that the contour of the electrodes becomes visible. Obviously, it is sufficient to fill this void with a non-conductive filling material having a refractive index n2 close to n1. These materials are generally non-conductive transparent oxides (NCTO), such as SiO2 or TiO2. Various methods have been employed to achieve this objective. However, they are unsatisfactory because the filling material may form beads outside the filling area or depressions liable to modify the path of the light rays, making the contour of the electrodes even more visible, as explained below.
FIGS. 1A and 1B depict diagrammatically a prior art method using the light-induced chemical vapour deposition (LICVD) technology as described, for example, by Wagner E. (STI, Micro-engineering 2003, EPFL: Lausanne). In a first step, depicted in FIG. 1A, a transparent substrate 5 is placed in an enclosure (not shown) into which a precursor gas, for example Sn(CH3)4 or SnCl4, is introduced in order to deposit a film of SnO2, which is a transparent conductive oxide (TCO) which forms the electrodes 1a. This deposition is effected by the LICVD process involving irradiation through a first mask 15 which is transparent to UV radiation in areas that correspond to the configuration of the electrodes to be obtained, which are separated by insulating spaces 3.
In a second step, depicted in FIG. 1B, the first mask 15 is replaced by a second mask 17 having a window transparent to UV radiation complementary to that of the first mask. Insulating spaces 3 are filled with non-conductive filling material 2. FIG. 1C depicts defects that can arise if the two masks are not superposed in a rigorously complementary manner. Either beads 4 of NCTO or depressions 6 may be produced that lead to localised modifications of the optical path and thus render visible some portions of the contour of the electrodes 1a. 
Defects of the same type may arise on filling the insulative spaces 3 with NCTO using the well-known lift-off technique. Beads 4 may then be produced on both edges 8 of the insulating space 3, as depicted in FIG. 2.